When the collection needs to come out.

When drainage is considered

Drainage is considered when symptoms, exam findings, or imaging show that the collection is harming the brain—or is likely to. New weakness, declining alertness, significant midline shift, or a collection that is enlarging on serial scans all raise the stakes.

Some small, stable collections are observed instead. Observation is a plan with return precautions and a follow-up scan, not a hope that the report was a false alarm.

Burr-hole drainage

A burr hole is a small opening in the skull. It is often used for chronic or liquid collections that can be washed out and drained through a modest aperture, sometimes with a temporary drain left in place. It is less exposure than a full craniotomy, which is why it is common in chronic subdural care.

Recurrence after drainage of a chronic collection is a known problem. That is one reason specialists now discuss adding MMA embolization.

Craniotomy

A craniotomy removes a larger piece of bone temporarily so the surgeon can reach solid acute clot, bleeding that cannot be controlled through a small hole, or a more complex collection. Acute subdural hematoma more often needs this kind of exposure.

Surgery plus MMA embolization

Drainage treats the mass that is already there. MMAE treats the membranes that can keep leaking. In EMBOLISE, among patients who already needed surgical evacuation of a subacute or chronic collection, adding MMAE lowered 90-day repeat surgery for recurrence or progression from 11.3% to 4.1%.

The 2026 SNIS guideline’s strongest recommendation is for that adjunctive use. Timing—before, during, or after drainage—is individualized.

What to expect

Hospital stay, drains, blood-thinner holds, seizure medicines, and activity limits depend on the operation and the person. Ask who will review the next scan, which symptoms mean a return to the emergency department, and whether MMAE is being considered as part of the same plan. The recovery guide covers follow-up in more detail.